A resistor is a two contact, no-junction monoelement device that serves to limit current flow. The resistance of a resistor is determined by the sheet resistance of the resistive region and the number of squares (.quadrature.'s) contained in the resistive region. The number of squares is found by dividing the length by the width. Therefore a 1 .mu.m.times.10 .mu.m resistor has 10.quadrature.'s while a 50 .mu.m.times.50 .mu.m resistor has 1.quadrature.. A polysilicon resistor is a strip of polysilicon doped with a material, such as N-type phosphorous (P.sub.31) or arsenic (As.sub.75), or P-type boron (B.sub.11), to be made more conductive. A normal polysilicon resistor has a sheet resistance, R.sub.s, less than 30 .OMEGA./.quadrature.(ohms/square) while a high polysilicon resistor has a sheet resistance, R.sub.s, greater than 100 .OMEGA./.quadrature.(ohms/square) (and generally less than 2k .OMEGA./.quadrature.(ohms/square)).
Due to the physical characteristics of polysilicon, there are many defects in the grain boundaries that make the polysilicon resistance change widely as the supply voltage changes as measured by the resistor's voltage coefficient of resistance (VCR), limiting the applications for polysilicon resistors. A resistor's VCR may be improved by adding to the resistor's dimensions, but this would increase the area of circuit design which is undesirable with the continued push towards increasing micominiaturization if semiconductor devices. Other attempts to overcome and avoid these defects have included using laser beams or similar high energy beams to anneal the deposited polysilicon.
For example, U.S. Pat. No. 4,560,422 to Patel discloses a polysilicon resistor with a reduced VCR by modifying the polysilicon to a polysilicon of lowered resistance by laser beam annealing. A metal reflective layer is deposited and etched to protect other areas, e.g. dielectrically isolated islands, from the laser beam annealing thus allowing formation of the desired resistor only in areas where formation is necessary.
U.S. Pat. No. 4,263,518 to Ballatore et al. discloses a method and apparatus for correcting the VCR of a resistor integral with a semiconductor body by using a metal conducting electrode layer formed on the dielectric and at least partially covering the resistive region of the resistor. The metal layer is brought to a suitable potential to produce equal and opposite variations in the resistance with respect to the ones created by the epitaxial layer, thus acting to ensure the resistance's linearity.
U.S. Pat. Nos. 5,489,547 and 5,554,873 to Erdeljac et al. each disclose a semiconductor device, and a method of forming same, having a p type polysilicon resistor with a moderate sheet resistance and low temperature coefficient of resistance formed by a double-level polysilicon process. A polysilicon resistor being of a first conductivity type and having a first temperature coefficient of resistance is formed over, and insulated from, a semiconductor layer. The polysilicon resistor is then doped with a dopant of a second conductivity type opposite the first conductivity type causing the polysilicon resistor to be of the second conductivity type and to have a second temperature coefficient of resistance that is closer to zero than the first temperature coefficient of resistance.
U.S. Pat. No. 5,640,137 to Mantha discloses a polysilicon resistor having additional structures that add to the polysilicon resistor's area and therefore reduce the self-induced temperature change caused by the voltage across the polysilicon resistor. Since the resistance depends upon the polysilicon resistor's temperature, the reduction of the self-induced temperature change lowers the variation in the resistance value at different voltages.
U.S. Pat. No. 5,391,979 to Kajimoto et al. discloses a constant current generating circuit including a high resistance element, normally turned on, for generating a very small current. Since the very small current is supplied from the high resistance element a constant current can be stably generated regardless of the change of the power supply voltage.